Steps to reproduce (admittedly a bit of work and maybe not needed from the description....)

1. Create a Thunderbird mail folder with many thousands of messages
2. Select that folder so the huge message list is loaded upon next launch
3. Quit Thunderbird
4. Launch Orca
5. Launch Thunderbird

(Wait forever see https://bugzilla.mozilla.org/show_bug.cgi?id=924915)

6. Quit Orca
7. In a python console type the following:

-----------------------
import pyatspi
d = pyatspi.Registry.getDesktop(0)
for child in d:
    print(child)
<press return here>
-----------------------

When I do this, in my environment I get:

[application | gnome-settings-daemon]
[application | ibus-x11]
[application | org.gnome.Software]
...
[application | gedit]
[DEAD]

Sometimes instead of [DEAD] it lists [invalid | ]. But either way, it's defunct. And dead/defunct application is Thunderbird.

8. With Thunderbird still running, in a terminal run an accessible pyatspi listener for "window" events. Here's mine:

-----------------------
#!/usr/bin/python3

import pyatspi

def listener(e):
    print(e)

pyatspi.Registry.registerEventListener(listener, "window:")
pyatspi.Registry.start()
-----------------------

9. Alt+Tab into and out of Thunderbird. I see things like:

window:deactivate(0, 0, Archives - my.email@foo.com - Mozilla Thunderbird)
	source: [invalid | ]
	host_application: [invalid | ]
window:activate(0, 0, Archives - my.email@foo.com - Mozilla Thunderbird)
	source: [invalid | ]
	host_application: [invalid | ]

10. Change the listener to listen for "object:state-changed:focused" events instead. Alt+Tab back into Thunderbird and arrow up and down in that huge list of messages. I see things like:

object:state-changed:focused(1, 0, 0)
	source: [window | ]
	host_application: [application | gnome-shell]
object:state-changed:focused(1, 0, 0)
	source: [invalid | ]
	host_application: [invalid | ]
object:state-changed:focused(1, 0, 0)
	source: [invalid | ]
	host_application: [invalid | ]
object:state-changed:focused(1, 0, 0)
	source: [invalid | ]
	host_application: [invalid | ]

Admittedly long way of saying that it appears that once the registry has concluded Thunderbird is defunct, it sticks with that conclusion. As a result, even after the huge pause of death (that mozilla bug referenced above), subsequent events emitted by Thunderbird are still coming from invalid/dead event sources and from an invalid/dead app. Thus Orca ignores them and Orca users are unhappy.

If this is a registry issue (Thunderbird is no longer dead, but the registry thinks it is), it would be super if periodic cleanup of applications could be done so Orca could eventually present Thunderbird once it becomes responsive again. Thanks!